geophysical monitoring of groundwater flow for mining and
TRANSCRIPT
Geophysical Monitoring of Groundwater Flow for Mining and Exploration ApplicationsGraham HeinsonSchool of Earth and Environmental SciencesUniversity of Adelaide, Adelaide SA 5005,
Mineral Exploration Through CoverAdelaide, Australia, Friday 27th June
Porous Rock Aquifers
Fractured Rock Aquifers
2 mBedding Plane
Uplift
1 m
Plan View
Bedding planes
Fracture
s
Hydraulic Conductivity,Heterogeneity
?
Figure adapted from Love, A. and Cook, P. (1999). The importance of fractured rock aquifers. Dept of Primary Industries and Resources, South Australia Report Book 99/23.
Aperture SizeFluid flow varies with cube of aperture diameter
Electrical current flow is linearly proportional to aperture diameter
Figure adapted from Love, A. and Cook, P. (1999). The importance of fractured rock aquifers. Dept of Primary Industries and Resources, South Australia Report Book 99/23.
Groundwater Flow
Figure adapted from Love, A. and Cook, P. (1999). The importance of fractured rock aquifers. Dept of Primary Industries and Resources, South Australia Report Book 99/23.
Porous Media Pump well
Observation well
Pump wellObservation
well
Porous media: assume homogeneous drawdown
Plan view Cross Section
Water Table
Fractured Media Pump well
Observation well
Pump well Observation well
Drawdown
Fractured media: heterogeneous drawdown
Water Table
Figure adapted from Love, A. and Cook, P. (1999). The importance of fractured rock aquifers. Dept of Primary Industries and Resources, South Australia Report Book 99/23.
Fractured Rocks in the Eastern Mt Lofty Ranges: 20 sites; lithology and structural geology
20 km
Borehole-Surface Resistivity
30 m
30 m
Surface Potential Maps
Orientation of bar – strike
Arrow – dip
Length of bar – degree of anisotropy
20 sites completed along eastern Mount Lofty Ranges
20 sites are being undertaken in western Mount Lofty Ranges
Electrokinetic PotentialResistivity gives amount of fluid (porosity) but does not tell us how well fluid flows (hydraulic conductivity)Electrokinetics: naturally occurring voltages that occur due to fluid movement
Salinity (number of charged ions in solution)pH (absorption of ions on to rock)Zeta potential (natural electrical double layer across rock boundary)
Electrokinetic PotentialFracture
< 1 m
m
Immobile fluid
Immobile fluid
Mobile fluid
Movement of ions in solution is known as an advective current
Electric double
layer
Electrokinetic PotentialHigh salinity → high concentration of ions in solutions, high advective currentpH → polarity of fluid ions absorbed onto fractures, polarity of advective current
Low pH (typically acidic <7) → positive ions attracted, hence advective current has higher density of negative ionsHigh pH (typically alkaline >7) → negative ions attracted, hence advective current has higher density of positive ions
Zeta potential → natural electric double layer of rock materials
Advective electric current
Fracture
Convective current (return path)
Advective current (fluid path)
V ia
ic Rrock and fluid
Rfluid
PCPV ∇=∇ησεζ
=∇
H'CHg
V ∇=∇ησ
εζρ=∇
Helmholtz-Smoluchovsky Equation
Darcy’s Law
HKAQ
∇−=
∇V = Electric potential gradient (V/m)∇P = Pressure gradient (Pa/m)∇H = Hydraulic gradient (m/m)ε = dielectric constant (F/m)ζ = ζ (zeta) potential (V)η = Fluid viscosity (Pa s)σ = Fluid electrical conductivity (S/m)ρ = Fluid density (kg/m3)g = Specific gravity (m/s2)C = Streaming potential coefficient (V/Pa)C’ = Streaming potential coefficient (V/m)
Q = Flow (m3/s)A = Area (m2)K = Hydraulic conductivity (m/s)∇H = Hydraulic gradient (m/m)
Governing Equations
Salinity term
tank manometers
sample tube
electrodes
data-taker
computer
ΔH
ΔV
Electrokinetics in the lab
Determination of ζ and C’
ζ = - 40mV C = -1.8 mV/mfor σ = 1470 μS/m
Sample: Q1B(fine sandstone)
(Wendouree)
Streaming Potential for 5 different hydraulic heads (sample Q1B)
-6
-5.5
-5
-4.5
-4
-3.5
-3
14:12 14:15 14:18 14:21
Time
SP (m
V)
H 1
H 2
H 3
H 4H 5
Linear relationship:V=Cs*H + constant
y = -0.018x - 5.3209R2 = 0.9992
-5.4
-5
-4.6
-4.2
-3.8
-3.4
-100 -80 -60 -40 -20 0
H (cm)
V (m
V)
Electrokinetic Field Measurements
36 electrodes in radial pattern, 6 every 5 m along a line, and with six lines spaced 60°Potentials measured every 5 s at each electrode relative to a distant reference.Transient changes in potential surrounding the pumped well of up to 10 mV are observed over time-scales of a few seconds after turning the pump on, to tens of minutes when equilibrium flow is established
V
Pump
30 m
Reference electrode
Switch on pump
1
2
34
56
Max
imum
flo
w d
irect
ion
Min
imum
flo
w
dire
ctio
n
Switch on pump
Line 1 Line 2 Line 3 Line 4 Line 5 Line 6
Negative potential change: change in pH?
Max
imum
flo
w d
irect
ion
Min
imum
flo
w d
irect
ion
1
2
34
56
Switch on pump
Almost no change in potential: low yield, or deep flow?
Interpretation
Water table
Before pumping… No flow, no electrokinetic potential!
Water table
Start pumping
Fluid flow near surface →large convective current ic → large potential
iaic
Water table
Late times
Deep flow → small convective current ic →small potential
ia
ic
ConclusionDownhole resistivity can indicate amount of fluid (porosity), and fluid connection paths Electrokinetic methods detect flow (hydraulic conductivity) in fractured and porous mediaPumped wells surrounded by electrodes can map fluid flow as a fucntion of time…..but processes are poorly understood, particularly salinity, pH and zeta potential